1. Field of the Invention
The present invention relates to a liquid crystal display device, particularly, a liquid crystal display device capable of keeping picture quality from deterioration caused by an AC driving method to achieve image display with high quality.
2. Description of the Related Art
A liquid crystal display module is used as a display device for a highly addressable color monitor of a computer and other information apparatuses or a TV set.
A liquid crystal display module basically includes a so-called liquid crystal display panel having a liquid crystal layer held between two (a pair of) substrates at least one of which is made of transparent glass. A voltage is selectively applied to various kinds of electrodes for forming a pixel, the electrodes formed on a substrate of the liquid crystal display panel, to switch a predetermined pixel on and off. A liquid crystal display module is superior in contrast performance and high-speed display performance.
FIG. 4 is a block diagram showing a schematic structure of a conventional liquid crystal display module.
The liquid crystal display module shown in FIG. 4 comprises a liquid crystal display panel 1, a gate driver part 2, a source driver part 3, a display controlling circuit 4 and a supply circuit 5.
The gate driver part 2 and the source driver part 3 are provided on the periphery of the liquid crystal display panel 1. The gate driver part 2 is formed from plural gate driver ICs provided on one side of the liquid crystal display panel 1. The source driver part 3 is formed from plural source driver ICs provided on another side of the liquid crystal display panel 1.
The display controlling circuit 4 performs timing adjustment for a display signal inputted from a display signal source (on a host side) such as a personal computer and a television receiving circuit so as to be suitable for display of the liquid crystal display panel 1 such as current alternation of data and converts the display signal into display data in a display form to be inputted to the gate driver part 2 and the source driver part 3 together with a synchronizing signal (a clock signal).
The gate driver part 2 and the source driver part 3 supply a scanning line with a scanning voltage on the basis of control of the display controlling circuit 4 and supply an image line with an image voltage to display an image. The supply circuit 5 generates various kinds of voltages necessary for the liquid crystal display device.
FIG. 5 illustrates an equivalent circuit of a pixel part of the liquid crystal display panel 1 shown in FIG. 4. FIG. 5 corresponds to geometrical arrangement of actual pixels. Plural sub pixels arranged in the shape of a matrix in a viewing display area (a pixel part) are formed from thin film transistors (TFTs), every one of which is used for one sub pixel.
In FIG. 5, DR, DG and DB denote image lines (also referred to as drain lines or source lines), G denotes a scanning line (also referred to as a gate line) and R, G and B denote pixel electrodes (ITO1) for respective colors (red, green and blue) Further, ITO2 denotes an opposite electrode (a common electrode), C1c denotes liquid crystal capacity equivalently indicating the liquid crystal layer and Cstg denotes holding capacity formed between a common signal line COM and a source electrode.
In the liquid crystal display panel 1 shown in FIG. 4, drain electrodes of thin film transistors (TFTs) of the respective pixels provided in a column direction are respectively connected to image lines (DR, DG and DB). The respective image lines (D) are connected to a source driver part 3 for supplying pixels provided in a column direction with an image voltage corresponding to display data.
Gate electrodes of thin film transistors (TFTs) of the respective pixels provided in a row direction are respectively connected to scanning lines (G). The respective scanning lines (G) are connected to a gate driver part 2 for supplying gates of the thin film transistors (TFT) with a scanning voltage (a positive or negative bias voltage) for one horizontal scanning period.
In displaying an image on the liquid crystal display panel 1, the gate driver part 2 selects the scanning lines (G0, G1, . . . Gj, Gj+1) from the upper part to the lower part (in the order of G0→G1) while the source driver part 3 supplies the image lines (DR, DG and DB) with an image voltage corresponding to the display data to apply the voltage to a pixel electrode (ITO1) during the selection period of a certain scanning line.
It is premised here that an operation is carried out in a so-called normally black-displaying mode in which the larger the image voltage supplied to the respective pixels is, the higher the luminance is.
A voltage supplied to the image line (D) is applied to the pixel electrode (ITO1) via a thin film transistor (TFT), and finally, holding capacity (Cstg) and liquid crystal capacity (Clc) are charged with electric charge and liquid crystal molecules are controlled to display an image.
The above-mentioned operation is described hereinafter with reference to a timing waveform.
FIG. 6 illustrates a voltage waveform outputted from the gate driver part 2 to the scanning line (G) and a voltage wavelength on an image line of an image voltage (VD) outputted from the source driver part 3 in a liquid crystal display module shown in FIG. 4.
A clock (CL1) shown in FIG. 6 is a clock for controlling output timing. The source driver part 3 outputs an image voltage (VD in FIG. 6) corresponding to the display data to the image lines (DR, DG and DB) from a point of falling time of the clock (CL1). FIG. 6 shows a voltage waveform of the image voltage (VD) in the case of displaying white.
The image voltage (VD) supplied to the image lines (DR, DG and DB) is AC-driven by switching the polarity between an image voltage with high potential with respect to a common voltage (VCOM) applied to the opposite electrode (ITO2) (referred to as an image voltage of the positive polarity (+), hereinafter) and an image voltage with low potential with respect to the common voltage (VCOM) (referred to as an image voltage of the negative polarity (−), hereinafter) for every horizontal scanning period (1H) in order to prevent the current voltage from being applied to liquid crystal capacity (Clc) in FIG. 5. FIG. 6 shows a case of using a dot inversion method, which is one of a common symmetry method, as an AC driving method.
On the other hand, a scanning voltage (VG) at a high level (referred to as an H level, hereinafter) is applied from the gate driver part 2 for one horizontal scanning period (1H) in the order of vertical scanning of the scanning lines (G0, G1, . . . Gj, Gj+1). Turning on, that is, selecting all the thin film transistors (TFTs) connected to the scanning line allows the image voltage (VD) outputted from the source driver part 3 to be applied to the liquid crystal capacity (Clc) and the holding capacity (Cstg).
Contrary to the above, in the case of the scanning voltage (VG) at a low level (referred to as an L level, hereinafter), all of the thin film transistors (TFTs) connected to the scanning lines (G0, G1, . . . Gj, Gj+1) are turned off, that is, not selected.
The waveform of the image voltage (VD) becomes dull in rising and falling processes of the image voltage (VD) in accordance with wiring resistance of the image lines (DR, DG and DB) and a time constant of the liquid crystal capacity (Clc), as shown in FIG. 6. Accordingly, the scanning voltage (VG) is changed from a voltage at the H level in a selection period to a voltage at the L level in a non-selection period after the image voltage (VD) is sufficiently saturated.
In the horizontal scanning period (N) in FIG. 6, for example, a slight difference in time (Tgd) is given from a point of time when the image voltage (VD) having the positive polarity is sufficiently saturated to a point of falling time of the clock (CL1) when the image voltage (VD) in a preceding horizontal scanning period (N+1) is outputted so as to change the scanning voltage (VG) from a voltage at the H level to a voltage at the L level.
Tgd is referred to as gate delay time in the following specification.
FIG. 7 is a simplified view simply showing pixel polarity and a pixel voltage level of a certain pixel in the case that white and black are alternately displayed for every vertical scanning period (referred to as a frame, hereinafter) in the conventional liquid crystal display module.
The pixel voltage shows a pattern that it is biased to a positive polarity side (a plus side) with respect to the common voltage (VCOM) and direct current is applied to the liquid crystal as an effective value when the image voltage changes in accordance with an AC cycle of the liquid crystal such as “white display” in negative polarity and “black display” in positive polarity, as shown in FIG. 7.
The pattern, especially, often occurs in the case of displaying a moving picture image and a direct current signal is always applied to the liquid crystal in the pattern. This causes deterioration in display quality as well as great deduction in life of the liquid crystal per se.
Further, display data in which white and black images alternately change in every frame often occurs in conversion from an interlaced (jump) scanning signal such as a television signal into progressive (sequential) scanning in liquid crystal driving. In the case of viewing a television image or a DVD image, which is displayed on the liquid crystal display module, for example, occurs a bias of a driving voltage of the liquid crystal. This causes deterioration in picture quality.
FIG. 8 shows pixel polarity in every frame in the case of inversing a phase of the pixel polarity in a certain fixed cycle (Period A and Period B) in the AC driving method shown in FIG. 7.
The pixel voltage in a first frame in Period A has a positive polarity (+) in accordance with a phase inversion signal shown in FIG. 8 while the voltage starts from the negative polarity (−) in Period B. Accordingly, the pixel polarity in the respective sections in Periods A and B is all opposite to each other such as positive (+) and negative (−) polarity.
Such an AC driving method is referred to as a phase inversion driving method in the specification hereinafter.
FIG. 9 is a simplified view simply showing pixel polarity and a pixel voltage level of a certain pixel in the case that white and black are alternately displayed in every frame in the phase inversion driving method.
As shown in FIG. 9, the pixel voltage biased to a negative polarity side (a minus side) with respect to the common voltage (VCOM) is to be biased to the positive polarity side (the plus side) after the inversion of the phase in accordance with the phase inversion driving method. As described above, carrying out an AC drive so that a bias of the pixel voltage would be on the positive polarity side or on the negative polarity side in a certain fixed cycle allows an effective direct current voltage applied to the liquid crystal to be reduced, as a result.
On the other hand, seeing the pixel polarity in the N-th frame and the pixel polarity in the first frame after a phase inversion switch, which are shown in FIG. 9, it is found that the positive (plus (+)) pixel polarity continues. In continuance of the same pixel polarity, there is sometimes a case of {(−)→(−)} or {(+)→(+)} in accordance with the timing of a switch in the phase inversion.
In the case of continuance of the pixel polarity, a condition for driving the liquid crystal (current alternation) is changed in appearance, so that flicker occurs on a display screen as a side effect.
The flicker occurs in the first frame with switch timing of the phase inversion signal shown in FIG. 8, that is, just after rising or falling of the phase inversion signal. As a result, the phase inversion drive has an effect of preventing the direct current voltage from being applied to the liquid crystal as well as a side effect of occurrence of flicker, which causes a problem that display quality is deteriorated, on the other hand.